1. Field of the Invention
The present invention relates to a liquid developer for use in electrophotography, in particular an improved liquid developer which is prepared by dispersing a toner consisting essentially of a pigment or dye and a specific non-aqueous resin dispersion uniformly in a carrier liquid having a high insulating property and a low dielectric constant.
2. Description of the Prior Art
It has been broadly utilized for developing a electrostatic latent image to make toner particles travel across the area to be developed and attached to the latent image area having a fixed polarity. Latent images may be formed by means of a variety of methods. The most popular method is one utilizing the surface of an electrophotoconductive plate whose insulating layer contains, dispersed and embedded therein, photoconductors such as zinc oxide and the like, which comprises electrifying the said surface by means of corona discharge or another process equivalent thereto, projecting light image on the surface for dissipating the electric charge present in the light-radiated area, and thus forming an electrostatic latent image.
In some cases, X-rays are utilized to bring about a change in the electric charge for the formation of the latent image, and in some methods another surface different from the photoconductive surface is treated according to the principle that the light image brings about a change in the electric charge to thereby form a similar latent image.
The thus electrostatic latent image-formed surface is developed by means of the following toner particle-containing suitable developer. Development has been usually effected by allowing powder-shaped dry toner particles or powder-shaped dry toner particles carried on coarse carrier particles to flow across the latent image-carrying surface and attaching toner particles to the latent image area. However, wet development using a liquid developer is now widely employed to meet the strong demand for high resolving power.
General liquid developers are prepared by incorporating toner particles in a carrier liquid having an insulating property to such an extent as not to destroy a latent image (wherein the electric resistance is more than 10.sup.9 .OMEGA..multidot.cm and the dielectric constant is less than 3), and according to some improvement achieved in this field, toner particles are provided with controlling surfaces for the control of toner particles per se.
However, such conventional liquid developers were defective in that because the resin and/or the polarity controlling agent constituting the toner diffuse and dissolve in the carrier liquid with the lapse of time, the polarity becomes indistinct, whereby the image density and the fixability are deteriorated, ground contamination is increased and so forth with the result being that copied images poor in sharpness are obtained. In addition, because the printing durability is low and the change of developers with the lapse of time is large there can not be obtained images having high concentration.
Still further, such conventional developers, once the toner agglomerates, are difficult to re-use, because it is impossible to re-disperse the agglomerated toner. Due to these deficiencies, such conventional liquid developers were not suitably used for offset printing or transfer such as charge transfer, press transfer, magnetic transfer and the like.